In 1923, Fischer and Tropsch, German chemists, developed a Fischer-Tropsch synthesis method (F-T synthesis method), and this method has enabled the production of liquid hydrocarbon from coal, natural gas, biomass and the like by way of syngas. A process producing liquid hydrocarbon from coal is referred to as a coal-to-liquid (CTL) process, a process producing from natural gas is referred to as a gas-to-liquid (GTL) process, and a process producing from biomass is referred to as a biomass-to-liquid (BTL) process, and recently, similar processes are collectively referred to as XTL (“X” resource-to-liquid) processes.
These processes first convert each resource material into syngas using methods such as gasification and reforming, and the composition of the syngas suited for XTL processes for producing liquid fuel preferably has a hydrogen-to-carbon monoxide ratio of approximately 2 as shown in the following Reaction Formula 1.CO+2H2+—[CH2]—n→—[CH2]—n+1H2O  [Reaction Formula 1]
(CO, 2H2, —[CH2]—n, and H2O represent carbon monoxide, hydrogen, hydrocarbon having chain length of n (number of carbons=n), and water, respectively.)
The ratio of hydrogen exceeding 2 is not preferable since it increases the selectivity on methane, and consequently, the selectivity on C5+ (hydrocarbon having 5 or more carbon atoms) relatively decreases. In Reaction Formula 1, olefin and oxygenate (a molecule including oxygen atoms such as alcohol, aldehyde, carboxylic acid and ketone) are also produced as by-products in addition to hydrocarbon having linear chains as shown above.
One of the main purposes of XTL processes is to obtain liquid fuels, and therefore, the latest trend is to decrease the selectivity on methane and to produce linear hydrocarbon, in particular, C5+ linear hydrocarbon with high selectivity by optimizing a selection of a reaction catalyst, a syngas ratio, a temperature, a pressure and the like. Herein, cobalt-series catalysts are normally used as the reaction catalyst, and such metal catalysts are used by being uniformly dispersed and supported on the surface of a support such as alumina, silica and titania. For the improvement of catalyst performances, noble metals such as Ru, Pt and Re may be used as a co-catalyst.
Such catalysts are normally used by being supported by a support such as alumina (γ-AlO3. α-Al2O3 and the like), silica (SiO2), titania (TiO2) and magnesia (MgO). However, the use of silica materials having mesoporous structures such as SBA-15 and MCM-41, and carbon-based materials having mesoporous structures such as CMK-3 and carbon nanotubes has also been expanded recently.
In general, an incipient wetness method, an impregnation method and the like are used for such supports when the catalysts are supported by the supports. For example, a target amount of the catalyst material is supported in the pores of the support while repeatedly performing processes of dissolving a cobalt salt of acid (Co (NO3)2.6H2O and the like), which is a catalyst precursor, and a salt such as Pt, Ru and Re used as a co-catalyst in proper solvents to prepare a mixed solution of the precursor, and impregnating the mixed solution of the precursor in the pores of the support, followed by drying. Next, the dried catalyst goes through a calcination process under air or inert gas atmosphere, and catalyst particles having a form in which cobalt oxide crystals are supported in the support are obtained. A Fischer-Tropsch cobalt catalyst shows an activity in a reduced metal state, and therefore, the catalyst has to go through sufficient reduction processes before reaction in every possible way. In a laboratory-scale experiment for developing catalysts, an in situ reduction method, in which the temperature is raised up to a reduction temperature while flowing reducing gas with a calcinated catalyst to be filled into a reactor, is normally used. However, commercial reactors often employ other methods since reduction temperatures are generally much higher than reaction temperatures, and separate reducing gas injection equipment is required for an in situ reduction method.
In most commercial processes, reduction is carried out by supplying reducing gas (a mixture of hydrogen and an inert gas where the hydrogen content is approximately 5 to 10%) with additional catalyst reduction equipment. Cobalt metals in a reduced state violently react with oxygen in air and are oxidized again. Therefore, a proper treatment is necessary to not expose cobalt metals to air, or to minimize the degree of oxidation when exposed. Such a treatment is referred to as passivation, and by an intentional mild oxidation of the surface only through the supply of a mixed gas (normally consisting of oxygen and an inert gas) with a low concentration of oxygen, the activity of a catalyst can be minimally degraded when exposed to air during its transfer.
However, the passivation method has several problems. First, the degree of proper passivation is very difficult to identify. The degree of oxidation treatment required for minimizing violent oxidation during the air exposure is different for each catalyst. In addition, there are problems that initial activity is not satisfactory since oxidation has been partially progressed before use, and activity is generally low compared to an in situ reduction method.
In order to solve such problems, S. Hammache et al. (refer to S. Hammache, J. G. Goodwin, Jr., R. Oukaci, Catalysis Today, 2002, 71, 361-367) designed a passivation method using CO gas or (CO+H2) gas. However, the method has a problem in that the activity of a catalyst is degraded due to the production of graphitic carbon on the surface of the catalyst, and additionally, the method further requires a heating equipment capable of being operated at high temperatures in a reactor since the reduction process includes treating the catalyst with hydrogen gas for 10 hours at a high temperature of 350° C. when activating a carbide compound catalyst.
In addition, F. Huber et al. (refer to F. Huber, H. Venvik, Catalysis Letters, 2006, 3-4, 211-220) proposed an encapsulation method using organic materials, a carbon layer coating method, a method of passivating metal catalysts through oxygen and N2O treatments. However, for activating, the method also requires reduction conditions of heating for 16 hours at a high temperature of 350° C. while supplying hydrogen gas.
Furthermore, various passivation methods carried out through the production of carbide and carbon have also been proposed. WO 03/002252 discloses a method for transferring or activating a catalyst by passivating the activated catalyst using a method of coating the surface of a metal precursor material supported in a support with carbon by adding a certain amount (5 to 20%) of short-chained hydrocarbon (methane, ethane, etc.) together with hydrogen gas, or introducing a syngas, in order to produce a carbide form of a metal catalyst in a hydrogen reduction process.
The metal catalysts having a carbide form are known to have increased activity after activation, and the activity is known to be further improved when a metal carbide form is formed in certain parts of an activated metal catalyst. However, WO 03/002252 discloses that hydrogen reduction treatment at a high temperature of 350° C. or greater is necessary to activate the catalyst passivated in a metal carbide form, thus requiring additional activation equipment in addition to the reaction equipment.
Meanwhile, even when an ex situ reduction method is used, a method without passivation by oxygen, that is, a method of introducing a catalyst directly into a reactor without being exposed to oxygen at all may be considered. However, the method also has problems. Catalyst reduction equipment and a reactor need to be relatively close, and the equipment may become larger since gas supply equipment, power, a heater and the like required for reduction all need to be included in a reactor system. In addition, there is a new challenging task on how to transfer solid particles from the catalyst reduction equipment to the reactor.
In order to solve these problems, Sasol Limited and the like have devised and used methods in which, by coating a reduced catalyst with wax or introducing the reduced catalyst inside a wax material, the transfer of the catalyst becomes simple while capable of blocking its contact with air (U.S. Patent Application Publication No. 2011/0301024). However, because wax materials are solid at room temperature, they need to be liquidified by heating in order to coat or insert catalyst particles, thus making the process complicated.